How NSAIDs are tested on Step 1

Questions cover why aspirin works for so long, why a COX-2 inhibitor still raises cardiovascular risk, which NSAID closes a PDA, and how NSAIDs injure the kidney or stomach.

workspace_premium Acetaminophen is not an NSAID

Acetaminophen inhibits COX mainly in the CNS. It reduces pain and fever but has no anti-inflammatory or antiplatelet effect and does not injure the stomach — its danger is hepatotoxicity.

Mechanism: the arachidonic acid pathway

Phospholipase A₂ releases arachidonic acid from membranes; COX converts it to prostaglandin H₂, the precursor of prostaglandins, prostacyclin, and thromboxane.

MembranelipidsPhospholipidsin cellmembranesArachidonicacidPhospholipaseA₂ (steroidsblock)COX-1,COX-2NSAIDs blockPGH₂CommonprecursorPGE₂,PGI₂,TXA₂Pain, fever,platelets
Arachidonic acid pathway. Blocking COX shunts arachidonic acid toward the lipoxygenase pathway (leukotrienes), which explains aspirin-exacerbated respiratory disease.
ProductMain actionEffect of losing it
PGE₂Fever, pain sensitization, gastric mucus, renal afferent dilationAntipyresis, analgesia — but ulcers, ↓ GFR
PGI₂ (prostacyclin)Vasodilation, inhibits platelets (endothelium, COX-2)Prothrombotic shift (COX-2 inhibitors)
TXA₂Platelet aggregation, vasoconstriction (platelet COX-1)Antiplatelet effect (aspirin)
PGE₁ (ductus)Keeps ductus arteriosus openIndomethacin closes a PDA

Low-dose aspirin (81 mg) selectively blocks platelet TXA₂ while sparing most endothelial PGI₂.

Aspirin, nonselective NSAIDs, and COX-2 inhibitors

The three groups differ in reversibility and selectivity.

Nonselective (incl. aspirin)Inhibit COX-1 and COX-2Antiplatelet effectMore GI ulcers and bleedingAspirin is irreversibleIbuprofen, naproxen, indomethacin,ketorolacCOX-2 selectiveCelecoxibSpare gastric COX-1 → fewer ulcersNo antiplatelet effect↓ Endothelial PGI₂ → ↑ MI and strokeriskSulfonamide — avoid in sulfa allergyvs
COX selectivity. Ibuprofen taken before aspirin can block aspirin's access to platelet COX-1 and reduce its cardioprotection.
AgentNotable use or feature
Aspirin (low dose)Secondary prevention of MI and stroke; Kawasaki disease (with IVIG)
IndomethacinPDA closure in preterm infants; acute gout; most CNS side effects
KetorolacIV/IM for short-term moderate-severe pain (≤ 5 days)
NaproxenLong half-life; lower cardiovascular risk among NSAIDs
CelecoxibArthritis in patients at high GI risk

NSAIDs reduce the effect of ACE inhibitors, ARBs, and diuretics, and raise lithium and methotrexate levels.

Adverse effects

Loss of protective prostaglandins drives most NSAID toxicity.

NSAID toxicityStomach↓ PGE₂ mucus,bicarbonate →ulcers, bleedingKidneyAfferentconstriction → AKI;interstitialnephritisHeart, vesselsNa⁺ retention, HTN,HF; ↑ MI riskAirwayLeukotriene excess →bronchospasm
NSAID toxicity. Kidney injury is most likely when renal perfusion depends on prostaglandins — heart failure, cirrhosis, CKD, volume depletion, or with ACE inhibitors and diuretics.
ProblemKey features
Aspirin-exacerbated respiratory diseaseAsthma, nasal polyps, and aspirin sensitivity (Samter triad)
Reye syndromeChild with viral illness (varicella, influenza) given aspirin → encephalopathy, microvesicular fatty liver
Salicylate toxicityTinnitus, vomiting, hyperventilation; early respiratory alkalosis then anion gap metabolic acidosis — treat with sodium bicarbonate, dialysis if severe
Renal papillary necrosisChronic heavy analgesic use

Misoprostol (a PGE₁ analog) or a PPI prevents NSAID ulcers; misoprostol is contraindicated in pregnancy.

Clinical application

NSAID physiology often appears in kidney questions.

functions Worked example — the triple whammy

A 74-year-old with heart failure takes lisinopril and furosemide. After a week of naproxen for knee pain, creatinine rises from 1.1 to 2.6 mg/dL. Explain.

  1. In heart failure, renal perfusion is low; prostaglandins dilate the afferent arteriole to protect GFR.
  2. Naproxen blocks prostaglandin synthesis, constricting the afferent arteriole.
  3. Lisinopril dilates the efferent arteriole, removing the other GFR-preserving mechanism.
  4. Furosemide reduces circulating volume further.

Answer. Hemodynamic acute kidney injury from afferent constriction (NSAID) plus efferent dilation (ACE inhibitor) plus volume depletion (diuretic). Stop the NSAID; use acetaminophen or topical NSAIDs for pain.